This application claims the priority of German application 196 53 756.8 filed Dec. 20, 1996 in Germany, the disclosure of which is expressly incorporated by reference herein.
The invention relates to a system and process for operating an NOx-accumulator for an internal-combustion engine.
For reasons of consumption, it is desirable to operate spark-ignited Otto engines and diesel engines at lambda &gt;1 during as many operating conditions as possible, in order to reduce the throttling losses during the charge cycle. The achievable lambda values are a function of the mixture preparation concept of the basic engine and, in the case of stratified charge engines or direct-injection engines, may extend up to an 6-fold air excess, that is, lambda=6. For operating conditions of this type, the known three-way catalysts are unsuitable because they operate at high conversion rates only at the operating condition lambda=1, that is, stoichiometric mixture and exhaust gas.
For the operation with an air excess, it is known to use NOx-accumulator catalyst combinations which, during the lean operation, store the NOx in front of the catalyst. For restoring the accumulating capacity of the NOx accumulators, regeneration phases are required with lambda &lt;1.
It is generally known to carry out the following steps for an engine timing in conjunction with an NOx-accumulation and the regeneration of the accumulator:
detection of the loading condition of the NOx-accumulator during the accumulation phase, and PA1 apportioning of a reducing agent as a function of the released NOx in the regeneration phase because the reducing agent, unless it is converted together with the NOx, emerges from the exhaust gas system and thus enters into the overall emission balance of the vehicle.
In the case of such an engine timing, the change-over event between the operation with the air excess (accumulator phase) and the operation with the fuel excess (regeneration phase) must not be noticeable by the driver. The known processes operate without exception by means of change-over operations which are controlled with respect to time and whose pulse-width rate, that is, the time relationship between the operating modes, is determined in a more or less high-expenditure manner by detecting the occurring NOx-mass. Furthermore, strategies are known which, by means of an NOx sensor, determine the breakthrough of the NOx accumulator and, as required, trigger a regeneration.
The known processes cause considerable matching expenditures, particularly when taking into account different load conditions. Furthermore, the service life of an NOx-accumulator operated by means of the known processes is limited.
In contrast to this state of the art, it is an object of the invention to provide a process for operating an NOx-accumulator for an internal-combustion engine which is simplified with respect to the determination of the time fractions of the individual operating modes and by means of which the service life of the NOx-accumulator is increased.
In this case, the invention is based on the recognition that the aging behavior of the NOx-accumulators is connected with the accumulating operation. Accordingly, the accumulating operation is connected with a volume change of the accumulated material which over time results in irreversible structural damage in the moderately elastic solid body. In addition, it was found that the NOx-conversion of an overall system formed of the NOx accumulator and the catalyst connected behind it is continuously reduced with an increasing filling of the NOx-accumulator so that NOx increasingly reaches the environment.
The present invention provides the replacing of the known "macroscopic" processes by a "microscopic" approach and determining the regeneration phase on the basis of individual operating cycles of the internal-combustion engine. The regeneration phases after approximately 30 seconds of lean operation which in the state of the art last approximately one second are replaced by an operating cycle with regeneration, for example, a rich operating cycle, after a sequence of operating cycles in which NOx is accumulated, such as lean operating cycles.
An operating cycle advantageously corresponds to a combustion cycle of the internal-combustion engine; that is, the smallest unit to be considered for the exhaust gas formation. Depending on the point of view, it is possible as an alternative to carry out the definition of an operating cycle also on the basis of other systems. Thus, it is possible, for example, to look at the internal-combustion engine as a whole and to define an operating cycle such that it extends from an operating stroke of the first cylinder of the internal-combustion engine to the next operating stroke of the first cylinder.
Different measures are suggested for regenerating the NOx accumulator. The apportioning of the regenerating agent by adjusting a fuel-air mixture with a fuel excess can take place very precisely. The apportioning of the regenerating agent by producing an engine cutout can be implemented particularly easily. The apportioning of the regenerating agent by the metering of fuel, in turn, can take place very precisely; in this case, there is the additional advantage that the regenerating agent does not take part in the combustion operation. Finally, it is suggested for apportioning the regenerating agent to provide a separate apportioning device for a reducing agent upstream of the NOx accumulator. As a result, other reducing agents than the fuel used for operating the internal-combustion engine, such a ammonia, can now also be used.
The number of operating cycles in which NOx is accumulated and which can be carried out before the operating cycle with the regeneration depends on the combustion process of the internal-combustion engine as well as the construction and the volume of the catalyst. This does not change the energy balance of the operation so that no consumption disadvantage occurs in comparison to the state of the art. The NOx accumulator is loaded only minimally, preferably essentially below 10% of its accumulation volume, which precludes the above-described aging operation. In addition, the catalyst connected behind usually operates in a filling condition in the case of which high conversion rates are ensured which are at the level of a three-way catalyst. Since, because of the high regeneration frequency, the probability is very high that, in the operating cycle with the regeneration, the same air mass flow rate will exist as in the operating cycle with NOx accumulating, a consideration of the emission masses is not necessary because the ratio of the NOx accumulation to the regeneration remains constant in all load conditions of the internal-combustion engine. In a particularly simple manner, it is therefore possible to let the operating cycle required for the regeneration follow a fixed number of operating cycles with NOx accumulation.
Slight deviations in the dynamics are compensated by the volume of the NOx accumulator which is large in comparison to the actual loading. In an operation with lambda=1 or richer, as occurs during idling for reasons of vehicle handling or during a full load for power reasons, the whole system is changed back into the condition with a completely emptied accumulator.
In preferred embodiments, the exhaust routing device between the outlet valve and the catalyst is to be designed such that a mixing of successive cylinder contents and thus a reduction of the concentration of the regenerating agent is precluded. This can be achieved by means of an elbow with a junction directly in front of the catalyst with constantly alternating regeneration cylinders. However, configurations are also conceivable in which the regeneration is always assigned to the same cylinder. There, it will be sufficient to carry out the exhaust routing to the catalyst separately for this cylinder. The invention can be applied to diesel internal-combustion engines as well as to Otto internal-combustion engines.
In an alternative or supplementary manner, it may be required in the case of internal-combustion engines with a large number of cylinder and a corresponding overlapping of the outlet phases of the individual cylinders, to use, instead of a single operating cycle for the regeneration, two or more operating cycles for the regeneration which follow one another in the ignition sequence in order to preclude a mixing of successive cylinder contents of different cylinders. This will correspondingly increase the number of intermediate operating cycles with NOx accumulation.
Finally, the number of operating cycles required for the regeneration also depends on the inertia of the NOx accumulator and the inertia of the catalyst so that, also for this reason, two or more operating cycles which follow one another in the ignition sequence may be required for the regeneration.